1. Field of the Invention
The present invention relates to a process for separating tertiary butyl alcohol from diisobutylene. More particularly the invention relates to a process utilizing pressure swing azeotrope distillation wherein two distillation columns are operated at different pressures to achieve the separation.
2. Related Information
Isobutylene is dimerized to diisobutylene according to the following reaction:(1) Isobutylene+IsobutyleneDiisobutylenes  (1)The dimerization of isobutylene with itself is of particular interest because either of the isomers of diisobutylene produce 2,2,4-trimethyl pentane (isooctane) when hydrogenated. The dimerization catalyst may include either an acidic cation exchange resin or zeolite. The dimerization of isobutylene over an acid catalyst in the presence of tertiary butyl alcohol is disclosed in U.S. Pat. No. 4,100,220.
Diisobutylene is an important oligomerization product useful for hydrogenation to isooctane. In one process isobutylene is oligomerized in the presence of tertiary butyl alcohol to produce the diisobutylene. Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 wherein gaseous olefins in the range of ethylene to pentene either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. The presence of tertiary butyl alcohol improves the selectivity to the dimer (diisobutylene) by suppressing further reaction to the trimer or higher. The tertiary butyl alcohol is then separated and recycled back to the oligomerization reactor. A process utilizing such a concept is disclosed in U.S. Pat. No. 4,100,220. However, diisobutylene forms a minimum boiling azeotrope with tertiary butyl alcohol rendering separation difficult. One common method of separating the two in the past has been by extractive distillation wherein a solvent or third component is introduced into the mixture to lower the relative volatility of one of the components. The draw back to this method is that a third component is used which itself must be separated from one of the two primary components.
It is an advantage of the present invention that the treatment of a product stream containing DIB and TBA with a third stream is avoided. It is a feature of the present invention that the TBA is recovered from a debutanizer DIB/TBA azeotrope by a separate lower pressure fractionation.